Concept:Symbiotic nitrogen-fixing bacteria live in the root nodules of leguminous plants.Explanation:Legumes form a mutualistic relationship with Rhizobium bacteria.These bacteria reside in root nodules and convert atmospheric nitrogen (N2) into compounds the plant can use.Azotobacter is a free-living nitrogen fixer, not a root-nodule symbiont.Clostridium and Nitrobacter are also not symbiotic root-nodule bacteria.Answer:D. Rhizobium